Five days after San Francisco's greatest display of civil disobedience since the Rodney King verdict, the most pressing question remains: What -- or who -- is Critical Mass?

Last Friday's massive traffic jam didn't result from hordes of war protesters, civil rights activists or gay rights marchers. It came from a source that would have seemed incongruous, even laughable, a year or two ago -- bike riders.

The event began in 1992, when a group of 25 riders decided, in the words of one Critical Mass historian, "to see how'd it feel" to ride in a large group during rush hour. One September evening, they rode from Justin Herman Plaza to the Marina Safeway. By the end of the ride, they found that an extra 20 cyclists had joined the ride.

A similar ride was held the following month, and each month after that. The word spread and the participants grew, reaching more than 5,000 last Friday. Critical Mass rides now take place in several cities across the country, including Seattle, Philadelphia, Portland, Ore., Austin, Texas, and London also has Critical Mass events.

Critical Mass riders say there is no stereotypical "member." Participants, they claim, are as diverse as San Francisco itself. The only thing they have in common is a love of bicycling and a desire to make the city safer for riding.

"The abusive element that everybody is making all the fuss about is very small," said Carlos Corujo, the owner of the Freewheel Community Workshop at Hayes and Ashbury, a membership bike shop that provides tools, components and expertise for people who want to repair their bikes.

"The profile of the typical participant is someone who commutes to work on a bike, and wants to do it safely and easily," Corujo said, an avid bicyclist whose whippet either romps next to his bicycle or crouches in a knapsack on Corujo's back, blinking into the wind.

Corujo has been riding in Critical Mass since its inception. And like many Massers, he is unrepentant about the event. The monthly ride is accomplishing its goal, he says -- more riders, all the time.

"I see the evidence myself every day," Corujo said. "The number of cyclists in this city has increased 10-fold since the first Critical Mass. People think about bikes when they see the ride, and many of them start riding."

By focusing on the radical elements linked to Critical Mass and not the larger issues behind the movement, cycling enthusiasts say, city officials and motorists will continue to endure Commuting Hell on the last Friday of every month.

"Critical Mass is the cyclists' way of expressing solidarity for one day a month," said Paul Dorn of the San Francisco Bicycle Coalition. "For one day, motorists understand how cyclists feel every day. There are 429,000 cars registered in San Francisco, and about 200,000 more come into the city daily. Critical Mass is just a lot of people saying they want a better transit system." Essential improvements, cyclists say, include bike paths -- where cars would be excluded -- striped bike lanes on roadways, abundant bike racks throughout the downtown area and centralized bike garages, where riders could leave their bicycles under secure conditions.

While those are the goals, city officials have expressed frustration because they cannot find leaders who can effectively speak for the organization. The reason is simple: Critical Mass is not an organization. It is a concept, an idea shared by thousands who act individually -- but in concert -- on a given time on a given day.

Critical Mass, in short, is a "meme" -- a term coined in 1978 by Richard Dawkins, an evolutionary biologist and essayist.

"Dawkins considers memes 'cultural DNA,' said Brad Wiener, an editor at Wired Books, the book publishing arm of Wired Magazine. Wiener, a cyclist who participated in Friday's Critical Mass, is also the author of an upcoming book on digital culture memes.

"Dawkins said that memes can act like microorganisms -- they look out for their own best interests by propagating," said Wiener. "And in that sense, Critical Mass has proved contagious. It has spread to other cities, and it continues to spread. It's definitely a meme -- an idea that spreads inexorably through a variety of ways."

Critical Mass is resistant to leadership, and is likely to remain so, said Wiener.

"When certain people attempt to speak for the group, it is evident to the rest of the people that they are speaking for their own benefit rather than the benefit of the group, so they are rejected," he said.

One such leader who apparently suffered rejection was C.J. Lackner, a Critical Mass participant who has plotted the routes for several earlier rides and had met with Mayor Willie Brown and Supervisor Michael Yaki to develop the sanctioned route that was ignored last Friday.

About 5,000 cyclists took to the streets on "outlaw" routes, hopelessly snarling traffic for hours. About 250 riders were arrested.By contrast, about 1,300 people were arrested in two days of rioting following the Rodney King verdict in 1992. During the Gulf War a year earlier, marches in downtown San Francisco sometimes exceeded 10,000 people. About 80 people were arrested during those demonstrations.

Lackner, however, bears no animus toward his fellow Massers for spurning his route.

"I think their rejection (of the approved route) is a clear demonstration of their frustration with the city's inaction on addressing crucial needs," said Lackner. "We've had plenty of summits and conferences already. The city knows our grievances -- now they have to do something."

Lackner said that punitive measures -- such as Brown's proposal to confiscate bicycles from Critical Massers who violate traffic laws -- will only inflame the situation.

"It will radicalize people even more," Lackner said. "They'll just be more careful about avoiding the police."

BIKE SPEAK

Critical Massers have their own vocabulary, as indicated by the following lexicon taken from a website popular with guerrilla cyclists:

-- Corking: As a group of cyclists enters an intersection, a few riders sprint ahead and stop in front of waiting cars, "corking" the thoroughfare until everyone has passed. This prevents motorists from inching forward, even when the light turns green. Many corkers hold up signs saying "Honk if you love bicycles," or "Thank you for waiting."

-- Organized Coincidence: The phenomenon of cyclists sponstaneously materializing each month at a specific site at a specific time, even though CM has no designated leaders or organizational structure. Individual responsibility for the event is thus avoided, but everyone can take credit.

-- Xerocracy: The practice of writing down ideas and xeroxing them for mass distribution at pre-ride gatherings. If enough people like your idea, it will be adopted. If not, remember -- there are no leaders in a xerocracy.

-- Going to Mass: Participating in a CM ride. One rider may query another: Are you going to Mass tomorrow?

-- Wheeze-In: A group activity consisting of several cyclists falling to the ground at an intersection in a mock display of asphyxiation by auto exhaust. An alternative to corking, it is equally effective in stopping traffic and infuriating motorists.

-- Defusing: Intervention by CM participants in confrontations between riders and motorists that look to be escalating dangerously.

-- Chain Reaction: The proliferation of CM events in cities across the country.